Apparatus and method for analysis of impurities in liquid carbon dioxide

ABSTRACT

Liquid CO 2  is expanded in specially designed primary and secondary nozzles to produce frozen solid CO 2 . The impurities initially contained in the liquid CO 2  are trapped inside of the frozen CO 2 , which is collected in a clean container. The container is emptied on top of a high purity surface. The impurities are concentrated on the surface, since these impurities are non-volatile and do not escape with the CO 2  gas formed as the frozen CO 2  is heated by the ambient temperature to its sublimation point. After all of the frozen CO 2  has sublimed, the surface can be analysed by standard analysis methods to determine quantity and composition of the impurities. The determination and quantification of non-volatile impurities, such as organic oils and greases, in the CO 2  supply that feeds, for example, cleaning systems utilizing CO 2 , is thus made possible.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus for analysing impurity levels in liquid carbon dioxide (CO₂), and to a method for using the apparatus.

2. Description of the Prior Art

Previous methods of separating impurities from liquid CO₂ rely primarily on High Pressure Liquid Chromatography (HPLC) to separate out the constituents of impurities contained in the liquid CO₂, for analysis of the impurity levels. The equipment necessary to carry out these methods is expensive and has an inherent lower resolution, quantification, and accuracy than that which is required for many applications. Also, some impurities may not be able to be separated using this technique, rendering the HPLC method useless for detecting the presence of these impurities. Furthermore, the equipment has to be calibrated frequently, usually just prior to use.

SUMMARY OF THE INVENTION

It is an object of the invention to mitigate and/or obviate the drawbacks of the prior art mentioned above, and provide an easy to use and inexpensive apparatus and method for determining the impurity levels in liquid CO₂.

In the invention, CO₂ snow (frozen CO₂) is used as a medium to determine non-volatile impurities in the CO₂ snow, and consequently determine the purity of the liquid CO₂. Liquid CO₂ is expanded in specially designed primary and secondary nozzles to produce low-density and low-velocity CO₂ snow. The impurities initially contained in the liquid CO₂ are trapped inside of the frozen CO₂, which is preferably collected in a clean container. According to one preferred embodiment, the container is emptied on top of a high purity surface. The impurities are concentrated on the surface, since these impurities are non-volatile and do not escape with the CO₂ gas formed as the CO₂ snow is heated by the ambient temperature to its sublimation point. After all of the CO₂ snow has sublimed, the surface can be analysed by standard methods to determine quantity and composition of the impurities. The determination and quantification of non-volatile impurities, such as organic oils and greases, in the CO₂ supply that feeds, for example, cleaning systems utilizing liquid CO₂, is thus made possible.

The system according to the invention has a primary nozzle and a secondary nozzle. The primary nozzle is connected to a liquid CO₂ source via one or more small orifices having a suitable shape, for example substantially round, a slit or an annulus shape (an annular slit). Liquid CO₂ is expanded after the orifices, and the expansion causes the liquid to freeze into snow particles. The primary nozzle is arranged to project the snow particles at a certain angle into the secondary nozzle. The snow particles collide with each other and the walls of the secondary nozzle, and the secondary nozzle geometry is designed to cause the snow to have multiple collisions with these walls. The collisions reduce the velocity of the snow and cause many snow particles to stick together further reducing the snow velocity. Thus, the snow is easily collected in a container for analysis. The container is emptied onto a high purity surface, and as the snow sublimes from the surface, the non-volatile impurities remain on the surface and accumulate in high concentrations.

The advantage of this system is the simplicity of use and low cost, since it does not require expensive analytical equipment. This simple and inexpensive method enables part-per-billion (ppb) resolution of non-volatile impurities.

Further features of the invention will be described or will become apparent in the course of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, the preferred embodiment thereof will now be described in detail by way of example, with reference to the accompanying drawing, in which:

FIG. 1 is a schematic side view of a system according to the invention,

FIG. 2 is a schematic elevational side view of the system shown in FIG. 1,

FIG. 3A is a schematic elevational side view of a first embodiment of the secondary nozzle,

FIG. 3B is a schematic elevational side view of a second embodiment of the secondary nozzle, and

FIG. 3C is a schematic elevational side view of a third embodiment of the secondary nozzle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIGS. 1 and 2, the system 1 according to the invention has a primary nozzle 10 and a secondary nozzle 20. The primary nozzle 10 has one or more small orifices 5 arranged upstream of the primary nozzle, and is connected to a liquid CO₂ source (not shown) via a valve (not shown). The orifices are of suitable shape for example substantially round, a slit or an annulus shape (an annular slit). A fitting 15, for example a VCR fitting, is arranged between the primary nozzle 10 and the liquid CO₂ source. Liquid CO₂ is expanded in the orifices 5, and the expansion causes the liquid to freeze into snow particles. The primary nozzle 10 is mounted to the secondary nozzle via a sealed opening 12 and is arranged to project the snow particles at a certain angle a into the secondary nozzle 20. The secondary nozzle has side walls 25, a bottom wall 26 and a discharge opening 27. The side walls form a closed cross-section and the bottom wall is sealingly fastened to one end of the side walls. Alternatively, the secondary nozzle 20 is fabricated from a solid piece of material, for example by milling and drilling. The snow particles collide with each other, and the side and bottom walls of the secondary nozzle 20. The secondary nozzle geometry is designed to cause the snow particles to have multiple collisions with the walls, examples of preferred embodiments are shown in FIGS. 3A to 3C. The collisions reduce the velocity of the snow and cause many snow particles to stick together, further reducing the snow velocity. The angle a is chosen so that a maximum number of collisions is obtained without clogging the secondary nozzle with frozen CO₂. A preferred angle is 45 degrees, but any angle between 1 and 89 degrees may be used. The cross-section of the secondary nozzle 20 is circular, oval, rectangular, quadratic or any geometry which will provide the necessary collisions. Thus, the snow is easily discharged through the discharge opening 27 and collected in a collection means 30 for analysis. The collection means is any suitable container, which can hold the frozen CO₂ without spilling or contaminating it. The collected snow is preferably deposited onto a clean surface (not shown) and, as the snow sublimes off the surface, the non-volatile impurities remain on the surface and accumulate in high concentrations.

The invention thus uses CO₂ snow as a medium to determine non-volatile impurities in CO₂ snow, and consequently determine the purity of liquid CO₂.

Since each of the primary and secondary nozzles are of fixed mechanical dimensions they can be calibrated to measure only one property, either mass loss of the bulk material or collection time, and a second measurement of the weight of the accumulated impurity or volume and density can then be used to calculate the exact concentration of impurities.

The method according to the invention preferably includes the steps of

connecting a liquid CO₂ supply to the primary nozzle,

expanding liquid CO₂ in the primary nozzle to produce frozen solid CO₂, which is projected further into the secondary nozzle, to produce low-density and low-velocity CO₂ snow; the impurities initially contained in the liquid CO₂ are trapped inside of the frozen CO₂,

collecting the CO₂ snow in a collecting means, which is any suitable clean container, such as a glass beaker,

preferably emptying the container, after collecting a sufficient quantity, onto a high purity surface, such as a polished silicon wafer, leaving the impurities concentrated on the wafer, since these impurities are non-volatile and do not escape with the CO₂ gas

analysing the surface by standard methods, after all of the CO₂ snow has sublimed to determine quantity and composition of the impurity.

Alternatively, the frozen CO₂ from the secondary nozzle 20 may be led directly to a real-time analysis means (not shown), for direct analysis. The primary nozzle 10, the secondary nozzle 20 and the real-time analysis means are preferably assembled as one unit, to facilitate field use.

Thus, the determination and quantification of non-volatile impurities, such as organic oils and greases, in the CO₂ supply that feeds, for example, cleaning systems utilizing liquid CO₂, is made possible. The user of the system according to the invention can easily verify the purity of the CO₂ prior to using it for cleaning operations.

EXAMPLE

A CO₂ cylinder, which was known to contain an impurity, was weighed, the analysis nozzles (primary nozzle and secondary nozzle) connected to it and 2 pounds of liquid CO₂ were expanded through the nozzle. A 1 liter beaker was filled with snow, inverted onto a Si wafer, and the snow was allowed to sublime.

The resulting, highly visible layer was then analysed by ellypsometry and weighing.

Four spots on the wafer were chosen for ellipsometer analysis: the centre, halfway to the edge, a thicker hazy spot and one spot near the edge. The thickness were recorded as follows: 141 Å, 163 Å, 260 Å, and 220 Å, respectively. On first order, the average of these values gives 196 Å±54 Å.

The total contaminated area can be calculated from the diameter of about 3.5″ or 9 cm, equalling 64 cm². The volume of this material is then 196×10-8 cm times 64 cm² equalling 1.25×10-4 cm³. Assuming a density of 0.8 g/ml for organic oils gives 1×10-4 g, or 0.1 mg.

The wafer was then weighed on a balance, rinsed with acetone, and weighed again; the net weight was 0.2 mg+0-0.1 mg. This contaminant was collected from 2 pounds of liquid CO₂. Thus, we can calculate 1×10-4 g divided by 1000 g equals 1×10-7, or 100 ppb.

Both methods calculated an impurity level of 100 ppb. Based on this and similar results, it is believed that contamination levels of less than 10 ppb can be visually detected and with appropriate instrumentation quantified. The procedure may be repeated multiple times to give yet higher degrees of resolution.

It will be appreciated that the above description relates to the preferred embodiment by way of example only. Many variations on the invention will be obvious to those knowledgeable in the field, and such obvious variations are within the scope of the invention as described and claimed, whether or not expressly described.

For example, contamination concentrated in the above described way, may be further analysed by standard analysis methods like Fourier Transform Infrared Spectroscopy (FTIR), Mass Spectrometry, Gas Chromatography (“HPGC”), Residual Gas Analysis (RGA), film thickness measurements etc., to determine the exact nature and proportions of the impurity. 

What is claimed as the invention is:
 1. A method for separation and determination of non-volatile impurities from liquid carbon dioxide includes the steps of: connecting a liquid CO₂ supply to a primary nozzle, expanding liquid CO₂ in said primary nozzle, to form frozen solid CO₂ and trapping the impurities initially contained in the liquid CO₂ inside the frozen CO₂, projecting the frozen CO₂ into a secondary nozzle, to produce low-density and low-velocity frozen CO₂, collecting the frozen CO₂ from a discharge opening in said secondary nozzle, heating the collected frozen CO₂, to its sublimation point, leaving the impurities concentrated since the non-volatile impurities do not escape with the CO₂ gas when the frozen CO₂ sublimes, analysing said impurities, after substantially all of the frozen CO₂ has sublimed, to determine the quantity and composition of the impurities.
 2. A method according to claim 1, wherein said step of collecting the discharged frozen CO₂ is performed by collecting the frozen CO₂ into a clean container, and wherein said step of heating the frozen CO₂ is performed by emptying said container onto a high purity surface, leaving the impurities concentrated on said surface when the frozen CO₂ sublimes from heating by the ambient temperature, followed by said step of analysing the impurities on said surface. 